Dawkins and Moran on neutral evolution (and me)

But he just came to mine. Larry Moran is a biochemist at the University of Toronto, and he runs a great blog writing a fair bit about evolution. He is on the side of Gould and Lewontin in the neutralist/selectionist debate in evolution. In the comments to a post from this morning, Dawkins, Darwin, Drift, and Neutral Theory, I asked for examples of traits that are known to have evolved via drift, rather than natural selection. Not that there aren't any traits that are thought not to be adaptive, but to make the point that we know that they are neutral just as much or little as we know others are adaptive, and that many traits must be adaptive, and that I think Moran is overemphasizing the importance of drift in evolution. Here's Moran's response:

It's shocking to me that you would have to ask such a question in 2011. As a graduate student interested in evolution you should be very familiar with the common examples and the debate.

Lewontin likes rhinoceros, I like tongue rolling. Some people can roll their tongues and some can't? Is this an adaptive trait?

I admit that these particular examples aren't going to change the course of evolution but it's all that's required to establish the real possibility that alleles with visible phenotypes can be neutral.

Once we've agreed on that point we can discuss whether the evolution of blood types in different human populations is due to selection or drift. (The answer is drift.)

Your other examples are more complicated but many of them arise from a combination of all the important mechanisms of evolution, not just natural selection.

I anticipate that you are going to fall back on the old canard that none of these traits have been proven beyond all reasonable doubt to be the result of random genetic drift acting on nearly neutral alleles, therefore it's permissible to say they all must be adaptations. If you don't see the flaw in that logic then there's no point in continuing.

I honestly didn't appreciate that much if at all, and neither did Richard Dawkins:

"Shocking"? Isn't that rather a patronising and arrogant response to Mr Østman?

Lewontin's guess that the difference between two-horned and one-horned rhinoceroses is non-adaptive is just that: a pure guess. Here is what Lewontin wrote:

"For example, the Indian rhinoceros has one horn and the African rhinoceros has two. Horns are an adaptation for protection against predators, but it is not true that one horn is specifically adaptive under Indian conditions as opposed to two horns on the African plains." (cited, together with my reply, in The Extended Phenotype, the chapter called 'Constraints on Perfection':

Actually, it seems unlikely that horns are an adaptation against predators. Lewontin is being too adaptationist here. Horns are more likely an adaptation against other rhinos, both as weapons and as displays. In which case it could very well be the case that one horn is best for Indian rhinos and two horns best for African rhinos (because a rhino has to match whatever is the norm for its competitors in its own region).

Never mind Lewontin's ignorance about the biology of rhinoceroses. The point is that Moran has no more right to assume, with Lewontin, that the number of rhinoceros horns is neutral than anybody else has the right to assume that they are under selection. Even if I could not invoke sexual selection (which makes it positively likely that Lewontin is wrong) Moran would have no right to assume that this is a good example of a neutral trait, still less would he have the right to patronise Bjørn Østman in that way.

Tongue-rolling is a polymorphism (I can 't do it, you very possibly can) which makes it more complicated. In this case we are not seeking an adaptive advantage in tongue-rolling per se. We are seeking some kind of balance between the two sides of the polymorphism. I don't know enough about it (nor does Moran) but I would keep an open mind, and the first place I would look would be for some kind of balancing selection (or perhaps heterozygous advantage) with tongue-rolling as a pleiotropic byproduct. Once again, the last thing I would do is simply assume in my ignorance (and Moran's ignorance) that selection is not involved.

7 comments:

Can I be shocked? "Evolved via drift" means "fixation of a trait via drift", and Larry says "some people can roll their tongue some can't"? So clearly it is not a trait that is fixed but rather a polymorphism as Dr. Dawkins points out. So maybe we should be shocked that it is Dr. Moran not Mr. Moran?

Yes, I did think of that, that the trait has not gone to fixation. However, even if it is truly drifting still, and not under balancing selection, the polymorphism may just have persisted in a rapidly expanding population.

Despite my status as an abject layperson, I wrote a rather long-ish post about this. In the end, my point is that just because you have two populations with two mutually exclusive traits that have gone to fixation in their respective population, and even if there is no clear selective advantage when comparing one trait with the other, that still does not necessarily imply a neutralist account. You can still call both traits "adaptive" and don't even necessarily have to invoke genetic drift. It's doubtless genetic drift plays an important role in evolution... I guess I'm just backing up Bjørn in that it's difficult if not impossible to argue convincingly that a trait which has gone to fixation is truly "neutral" -- only I'm taking a somewhat philosophical approach to it.

Larry is not the greatest spokesperson for the neutralist/pluralist side, IMO. I can't seem to get along with him while AGREEING, and he's about ten times more condescending to his opposition.

I think it's utterly meaningless to debate the neutrality or adaptiveness of complex traits like tongue rolling and limbs or whatever. Too many genetic and epigenetic processes involved, and we don't even really understand how it works yet. At that hierarchical level, at least three of the four fundamental 'forces' (drift, selection, mutation; recombination is optional in that context) are DEFINITELY involved. It's impossible to treat these things as a single evolutionary entity, or at least extremely foolish to do so.

Much of mainstream evolutionary theory was formulated by ecologists in the 60's and 70's, who had very little understanding of how these macroscopic features actually operated on a more molecular level – since molecular biology was still in its early infancy. Modern molecular biology has undoubtedly revolutionised evolutionary biology, though many ecologists strangely refuse to acknowledge the fact. the result is this vast and ever-growing rift between molecular/cellular/genetic and ecological/behavioural evolutionary biologists – so bad that we even often use terminology very differently. I think at least some fuel for our seemingly irreconcilable debate comes from that divide.

You do see the world quite differently depending on your field. Where I come from, one would be insane to argue everything is streamlined and adaptive; the non-adaptive phenomena are bloody obvious. To an ecologist, perhaps the opposite is true.

Of course, genomes are not all there is to an organism, and there is therefore definitely a need for reconciliation eventually. Our camp seems to be working from a bottom-up approach, first sorting out some solid principles for genetic and molecular evolution, and gradually working up the hierarchical levels. Some people are working on the top-down approach (eg. molecular ecology, arguably evo-devo, though it's still more devo than evo these days).

Lastly, as I argued on Larry's blog, much of 'modern synthesis' (ie, central tenets of evolutionary ecology) is based on a rather phylogenetically limited view of the world – things an ecologist could watch in the field, preferably things that moved. Very strong bias towards large metazoa. I don't blame them – only recently are we gradually building up the tools to study ecology where the bulk of it really happens – the microbial world. But what works for a pride of lions may not necessarily work for a bacterial cytoplasmic endosymbiont of a ciliate. There are definitely principles common to all, but we are unlikely to find and formulate them well by looking at solely large vertebrates, or even metazoa as a whole (<<1% of total biodiversity).

While these neutralism-adaptationism discussions are fun in a friendly environment, they are somewhat meaningless if taken too seriously. Before arguing over evolution as a whole, we must wait for the reconciliation of small-scale and large-scale subdisciplines of evolutionary biology, and a proper integration of real phylogenetic diverisity. The Modern Synthesis and Neo-Darwinism seem to fail at both, though to be honest, I can't for the life of me figure out what either of those really are, nor can some of the evolutionary biologists I talk to. I think evolutionary biology is still too young for such rigid tribes and labels.

-Psi-(not a PhD, nor a BSc grad yet – does that matter? ;p)

PS: For clarity, I prefer to use "non-adaptive" rather than "neutral" – most "neutral" models do incorporate selection, though 'negative' rather than 'positive'. What they omit is adaptation – via 'positive' selection. (must thank Mike Lynch for making that distinction clear to me...) Nor is selection synonymous with adaptation, but I think some might mix those up too.

Yes, I appreciated your comment on Sandwalk. I agree with your general idea that how we view evolution depends on where we are coming from. Biochemists vs. ecologists, etc. Different researchers aim to explain different things, and that sometimes gets lost in the discussion - there are different mechanisms operating between bacteria and mammals, for sure. However, theoretical models are meant to apply everywhere, at least at times, and of course even elephant evolution is driven by changes at the molecular level.

(not a PhD, nor a BSc grad yet – does that matter? ;p)

No, it doesn't. But you too would likely object if I said you were a high school student with a side interest in evolution, no?

Pleiotropy comes from the Greek πλείων pleion, meaning "more", and τρέπειν trepein, meaning "to turn, to convert". It designates the occurrence of a single gene affecting multiple traits, and is a hugely important concept in evolutionary biology.

I'm a postdoc at UC Santa Barbara.

All Many aspects of evolution interest me, but my research focus is currently on microbial evolution, adaptive radiation, speciation, fitness landscapes, epistasis, and the influence of genetic architecture on adaptation and speciation.